Skin aging is a multifactorial process driven by both intrinsic (chronological aging) and extrinsic (environmental) factors. The primary environmental factor that damages human skin to cause premature aging is UV irradiation from the sun. Sun-induced skin aging (“photoaging”), like chronological aging, is a cumulative process. However, unlike chronological aging, which depends solely on the passage of time, photoaging is a function of the degree to which one is exposed to the sun over a lifetime. The photoaging process is cumulative, with UV irradiation invoking a complex sequence of molecular responses that progressively damage the skin.
Signs of photoaged skin include wrinkles, pigmented lesions such as freckles, lentigines, and patchy hyperpigmentation. Other signs include a loss of tone and elasticity, increased skin fragility, areas of purpura, keratoses, and telangiectasis. Epidermal atrophy and discrete changes in collagen and elastic fibers characteristically occur. In severely photoaged skin, the collagen fibers are fragmented and thickened. Elastic fibers also appear fragmented and may exhibit progressive cross-linkage and calcification. These alterations in collagen and elastic fibers have been demonstrated to worsen with continued exposure.
The most effective measure to prevent photoaging is total avoidance of UV irradiation. Since total avoidance of sunlight is an impractical solution, blocking UV irradiation with protective clothing and/or sunscreen is a common, although not entirely satisfying, alternative. Such protective measures can help prevent future UV damage, but they do nothing to remedy past damage.
Various topical agents have also been used in an effort to treat photoaged skin. Clinically proven to not only reverse but also to prevent the signs of photoaging, retinoids are a class of materials widely recognized as effective. Retinoids are natural and synthetic compounds derived from all-trans-retinol, commonly known as Vitamin A. Retinol is transported to various cells where it is stored as a Vitamin A esters or oxidized sequentially through a retinaldehyde intermediate to all-trans-retinoic acid. Retinoids are important for embryonic development, the survival of lymphocytes, normal vision, and epithelial tissue differentiation. As such, they are critical for normal tissue function and human survival.
Although they can be very effective, retinoids are not well tolerated by many people. Common side effects associated with topically applied retinoids include burning and stinging of the skin, peeling, redness, and heightened photosensitivity. Furthermore, the therapeutic effect decreases over time, necessitating the use of increasingly higher retinoid levels to maintain the same level of benefit. Retinoid side effects are largely dose-dependent. As a result, many individuals with a sensitivity to retinoids cannot tolerate levels sufficient to provide the desired positive results. Additionally, retinoids are rapidly metabolized by the body, resulting in the need for higher doses than would otherwise be required to achieve the desired therapeutic effects.
Accordingly, there still remains a need for alternative compositions and methods that can prevent and/or reverse the signs of photoaging.